fix(home-manager): allow overriding styles for the rofi theme

[aloop]

Fix for #122.

Alters programs.rofi.theme to use an attrset instead of a path directly, so that users can add overriding styles to programs.rofi.theme.

A minimal example of how to change the font used in the theme would look like the following:

{
  programs.rofi = {
    enable = true;

    catppuccin.enable = true;

    theme."*".font = "monospace 14";
}

I've tested the changes with my NixOS/home-manager configuration, and it seems to work fine.

[getchoo]

tysm!